DE69125070T2 - System for the online determination of the degree of alloy in annealed steel sheets - Google Patents

Description

The present invention relates to a system for non-destructive and continuous on-line determination of the alloying degree in annealed galvanized steel sheets using X-ray diffraction, wherein the annealed galvanized steel sheets are produced by applying heat treatments just after galvanizing.

Up to now, annealed galvanized steel sheets, in which paintability, paint adhesion properties and weldability are imparted in addition to the corrosion resistance of galvanized steel, have been produced with wide application ranges. These annealed galvanized steel sheets have been produced by subjecting steel sheets to a continuous treatment of hot-dip galvanization, electrogalvanization or zinc vapor deposition and then to a post-heat treatment, whereby the galvanization and the base steel are alloyed together.

When the steel sheets are heat treated after galvanizing in the usual manner as described above, the η-Zn phase in the coating disappears as the alloy progresses by diffusion of Fe and Zn, and the - (FeZn₁₃), δ₁-(FeZn₇) and Γ(Fe₅Zn₂₁) phases grow successively.

So far it has been said that the quality of annealed galvanized steel sheets has a close relationship with the degree of alloy formation. If the degree of alloy formation is so low that a relatively soft thick layer of the -phase remains on the surface of the coating layer, it is so increased in surface friction with a mold (die) during press forming that the supply into the mold becomes poorer and it is preferable to reduce the proportion of the -phase. However, in view of the paintability and cosmetic corrosion resistance of annealed galvanized layers thereon, the proportion of the -phase should be maintained. On the other hand, when the degree of alloying is so high that a hard but brittle thick layer of the Γ-phase grows between the coating layer and the base steel, "chalking" occurs, a phenomenon that the Γ-phase of the coating layer peels off in powdery form. When this chalking phenomenon occurs to some considerable extent, not only is the press forming adversely affected, but the corrosion resistance of the annealed galvanized steel sheets becomes poor because the coating layer substantially, if not completely, disappears.

Therefore, in order to produce annealed galvanized steel sheets of improved quality, it is necessary to control the degree of alloying, thereby controlling the growth of the Γ phase to leave an appropriate amount of the - phase on the surface of the coating layer.

To determine the degree of alloying of annealed galvanized steel sheets, various methods have been used so far in this field, as mentioned below.

The Patent Abstracts of Japan, Vol. 008, No. 208 (P-302), September 21, 1984, show a method for measuring the degree of alloy formation on a production line without regard to the fluctuation of the measuring conditions, by directing parallel X-rays at an annealed galvanized steel sheet and detecting the intensity ratio of the diffracted X-rays from two specified crystal faces. The measuring device comprises the X-ray tubes 12, detectors 20, 24, 28 for the - and δ₁- phases and the background and an operating device 30 for calculating the degree of alloy formation.

The simplest method of all involves a visual or photometric determination of a color tone change in the surface of the coating layer just after galvanizing and annealing or a visual determination of the amount of coating layer peeling off a sample in a bending/rebending test, the so-called chalking test, but it is inaccurate. Chemical analysis of the average Fe content in the coating layer of a test piece is also available, which has been traditionally used as an index of alloying grade. However, a problem with this chemical determination is that it requires a lot of time from sampling to completion of the analysis, which involves a certain time delay in tracing the result back to the heat treatment in the alloying furnace.

To determine the degree of alloying from the structure of a coating layer for this purpose, a sample may be polished in cross-section to observe the cross-section of the coating layer under an optical microscope or a scanning electron microscope and thereby measure the thickness of the -, δ₁- and Γ-phases. Alternatively, a sample may be measured for its X-ray diffraction profiles with an analytical X-ray diffraction device, whereby the degree of alloying is determined by the X-ray diffraction intensities of the -, δ₁- and Γ-phases. These methods are preferred in view of providing a determination of the degree of alloying from the structure of the coating layer. However, a problem with these methods is that they require a long time from sampling to completion of analysis, causing some time delay in tracing the results back to the heat treatment in the alloying furnace. A problem common to all of the above-mentioned chalking tests, chemical analysis, cross-sectional observation and X-ray diffraction analysis is that because of their destructive nature, it is impossible to provide a determination of the alloying level of annealed galvanized steel sheets along the entire width or in the direction of the production line.

On the other hand, X-ray diffraction methods have been proposed to make an on-line, non-destructive and continuous determination of the alloying degree. For example, Japanese Laid-Open Patent Application No. 61-148355 shows a method for determining the average content of Fe in a coating layer by measuring the X-ray diffraction intensities of the Γ phase with an interplanar spacing of d = about 1.22x10-10 m (1.22 Å) and the α phase with an interplanar spacing of d = about 1.44x10-10 m (1.44 Å) and substituting the two measurements for a functional equation for the average content of Fe in the coating layer using the variables of the precalculated X-ray diffraction intensities of the α-Fe and Γ phases. With this method, in which what is measured is limited to the Γ phase, it is impossible to measure the amount of δ and δ1 phases that have been formed and to find the structure of the coating layer accurately.

With these conventional methods for determining the alloying degree by X-ray diffraction, it is also impossible to measure X-ray diffraction intensities accurately because the detector is fixed for the diffracted X-rays of the 2θ position (diffraction angle) of a specific lattice plane of the Fe-Zn intermetallic compound phases to be measured. In other words, the -, δ₁- and Γ-phases are all non-stoichiometric compounds, which vary in the content of Fe depending on the degree of diffusion of Fe and Zn, as can be seen from an equilibrium state diagram in which the - phase has a range of Fe content from about 5.5 wt.% to about 6.2 wt.%, the δ₁ phase has a range of Fe content from about 7.0 wt.% to 11.4 wt.%, and the P phase has a range of Fe content from about 20.0 wt.% to about 28.0 wt.%. Depending on the degree of alloying, there are thus changes in the interplanar spacing of the -, δ₁- and Γ phases and accordingly results in the change in the 2θ positions of the X-ray diffraction peaks of the respective phases. This phenomenon, which has been experimentally confirmed by the inventors of the present invention, will now be explained in more detail with reference to Figure 1, which shows the X-ray diffraction profiles of annealed galvanized coatings, each having a coating weight of about 45 g/m², which were heat-treated in a salt bath at 500°C for 5 seconds and 60 seconds for alloying, using a Cr tube (operated at a tube voltage of 40 kV and a tube current of 70 mA). In Figure 1, reference numeral 1 represents the X-ray diffraction profile of the annealed galvanized coating obtained by heat treatment at 500°C for 5 seconds, and 2 that of the annealed galvanized coating obtained by heat treatment at 500°C for 60 seconds. As understood from Figure 1, the X-ray diffraction peak peaks of the -, δ₁ and Γ phases of the annealed electroplated coating heated at 500°C for 5 seconds are located at 2θ = 130.0°, 2θ = 126.0° and 2θ = 139.0°, respectively, but those of the annealed electroplated coatings having an increased alloying degree because they were heated at 500°C for 60 seconds are found at slightly higher positions, e.g., 2θ = 130.5°, 2θ = 127.0° and 2θ = 139.5°, respectively. As an example, assuming that the alloying degree at the X-ray diffraction intensity of the δ₁ phase with an interplanar distance of d = 1.28x10⊃min;¹⊃0;m (1.28 Å), it is then found that with a diffracted X-ray detector fixed at the apex position of the X-ray diffraction peak of the δ1 phase of the annealed galvanized coating heated at 500°C for 5 seconds, e.g. 2θ = 126.0°, the X-ray diffraction intensity of the δ1 phase of the annealed galvanized coating which has an increased degree of alloying because it was heated at 500°C for 60 seconds is measured to be about 14,000 (cps = counts per second) at this position. In other words, the measurement is incorrect by as much as about 8,000 (cps) because the apex position of the peak of the latter annealed galvanized coating at 2θ = 127.0º, where the X-ray diffraction intensity is about 22000 (cps). Therefore, if the diffracted X-ray detector is fixed at the 2θ position of a specific lattice plane, the apex position of the peak changes with the change in the alloying degree and deviates from this detector, thus making it impossible to measure the X-ray diffraction intensity, ie, the alloying degree, constantly and normally.

As already stated, the -, δ₁ and Γ phases are all non-stoichiometric compounds, each of which has a certain range of Fe content. Although it depends on the alloying conditions applied and the type of base steel to be coated, this gives rise to variations in the Fe contents of the -, δ₁ and Γ phases, and in turn causes the average Fe content of the coating layer to vary even when the -, δ₁ and Γ phases are formed in the same amounts. Thus, it is impractical to provide the accurate determination of the average Fe content in a coating layer by X-ray diffraction.

As mentioned above, X-ray diffraction methods can be effectively used for on-line, non-destructive and continuous determination of the alloying degree. However, with conventional methods in which, as mentioned above, the diffracted X-ray detector is fixedly arranged at the 2θ position of the specific lattice plane of interest, it is impossible to measure the X-ray diffraction intensity constantly at the apex position of the X-ray diffraction peak, only to reduce the accuracy of the measurement and obtain information about the amounts of formation of the -, δ₁ and Γ phases or the structure of the coating layer.

An accurate determination of an X-ray diffraction intensity at the apex position of an X-ray diffraction peak can be achieved by measuring the associated X-ray diffraction profile. The X-ray diffraction profile is conventionally measured by scanning a detector with respect to an X-ray tube within a certain 2θ range according to the θ-2θ scanning method. However, if one wants to make an on-line determination of the X-ray diffraction, a lot of time is required for scanning the detector during which the alloying degree fluctuates, making it impossible to measure the X-ray diffraction profile accurately.

The present invention seeks to overcome the above-mentioned problems of the prior art by providing a system for achieving a non-destructive, continuous and accurate on-line determination of the alloying level in annealed galvanized steel sheets.

When galvanized steel sheets are heat-treated to form alloys, the -, δ1 - and Γ-phases are successively formed in the coating layer, as mentioned above. With the progress of alloy formation, the -phase disappears, but the Γ-phase grows and becomes thick instead. These phenomena are shown in Figure 2, which shows the X-ray diffraction profiles measured by X-ray diffraction, and are also shown schematically in Figure 3.

The 3 curves shown in Fig. 2 represent the X-ray diffraction profiles of the electroplated annealed coating, each having a coating weight of about 45 g/m2, which was heat-treated in a salt bath to form an alloy. Curve 3 represents the X-ray diffraction profile of the electroplated annealed coating which was heat-treated at 500°C for 5 seconds to form an alloy, and the structure of its coating layer is schematically outlined in Fig. 3a. Curve 4 represents the X-ray diffraction profile of the electroplated annealed coating which was heat-treated at 500°C for 30 seconds to form an alloy, and the structure of its coating layer is schematically outlined in Fig. 3b. Curve 5 shows the X-ray diffraction profile of the galvanized annealed coating heat treated at 500°C for 60 seconds to form an alloy, and the structure of the coating layer is schematically outlined in Fig. 3c. With further reference to Figs. 2 and 3, it can be seen that the longer the heat treatment time and the higher the alloying level, the smaller the volumetric fraction of the α-phase and the lower the X-ray intensity. In contrast, the increase in the volumetric fraction of the r-phase results in an increase in the X-ray diffraction intensity. The X-ray diffraction intensity of the δ1-phase increases as the heat treatment time increases from 5 seconds to 30 seconds, but decreases from 60 seconds after the heat treatment. This is because as alloy formation progresses, the volumetric fraction of the -phase decreases with an increase in the volumetric fraction of the δ1 phase; however, further alloy formation progress causes a decrease in the volumetric fraction of the δ1 phase due to an increase in the volumetric fraction of the Γ phase.

In addition, as the alloying degree increases with an increase in the heat treatment time, the peak positions the X-ray diffraction peaks of all the -, δ₁- and Γ- phases are shifted towards slightly higher 2θ-positions.

Taking these phenomena into account, the inventors have accomplished an X-ray diffraction system for achieving a non-destructive, continuous and accurate on-line determination of the alloying degree of annealed galvanized steel sheets from the X-ray diffraction intensities of the - and Γ phases, and preferably the δ1 phase, which vary with a change in the alloying degree, at the apex positions of the X-ray diffraction peaks.

The above and other objects and features of this invention and the manner of attaining them and the invention itself will be apparent and best understood by reference to the following description of the embodiment of the invention taken in conjunction with the accompanying drawings, in which:

Figure 1 is a graph showing the X-ray diffraction profiles of the hot-dipped galvanized annealed coating, each at a coating weight of about 45 g/m², heat treated on a salt bath at 500ºC for 5 seconds and 60 seconds.

Figure 2 is a diagram showing the profiles of the hot-dipped galvanized annealed coating, each with a coating weight of about 45 g/m², heat treated in a salt bath at 500ºC for 5 seconds, 30 seconds and 60 seconds,

Figure 3(a), (b) and (c) are schematic sketches showing the structures of the coating layers corresponding to the three X-ray diffraction profiles of Figure 2,

Figure 4 is a schematic view of an embodiment of the system according to this invention,

Figure 5 is a schematic view of the internal structure of the measuring unit,

Figure 6 is a diagram showing the X-ray diffraction profiles of a hot-dipped galvanized annealed coating each comprising galvanized annealed steel sheets produced by a continuous galvanizing and annealing production line at a line speed of 100 m/min. and having a coating weight of about 45 g/m2, which are heat treated at the respective alloying temperatures of 480ºC, 520ºC and 600ºC in a galvanized sheet annealing furnace,

Figure 7 is a schematic sketch showing how to test the drawing properties, and

Figure 8 is a schematic sketch showing how to test the chalking property.

The present invention will now be explained in more detail with reference to Figs. 4 to 8.

Referring first to Figure 4, there is shown schematically an embodiment of the system according to the invention for on-line determination of the alloying degree in galvanized annealed steel sheets. As shown, the system according to this invention is composed of a measuring unit 6, an X-ray generator 7, a high voltage wave 8, a control unit 9 for controlling the operation of the X-ray generator and the detector scanning, an arithmetic unit 10 for receiving the calculation results of the X-ray diffraction intensity obtained by the associated detectors, a cooling unit 11 for cooling e.g. an X-ray tube, a control unit 12 for displacing the measuring unit in the width direction of the galvanized annealed steel sheet, a chamber 13 for shielding the X-ray radiation and two pairs of clamp rollers 14 for preventing fluctuations of the galvanized annealed steel sheet and leakage of X-rays. The galvanized annealed steel sheet is shown at 15. The measuring unit 6 is designed to be displaced in the width direction of the galvanized annealed steel sheet 15 by a displacement device, not shown. The shield chamber 13 is controlled at a constant internal temperature by an air conditioner 16. This is because a change in the internal temperature of the chamber 13 gives rise to a change in the air density, which otherwise leads to a change in the absorption of X-rays by air and a deterioration in the accuracy of the measurement.

Referring to Figure 5, a schematic diagram showing the internal structure of the measuring unit 6 used with the present system, reference numeral 17 denotes an X-ray tube for generating X-rays, 18 a detector for measuring background intensity, 19 a detector for measuring X-ray diffraction intensity of a Γ phase, 20 a detector for measuring X-ray diffraction intensity of a -phase, 21 a detector for measuring X-ray diffraction intensity of a δ1 phase, 22 incident X-rays, 23, 24, 25 and 26 diffracted X-rays from the background, Γ, - and δ1 phases, 27 a solar slit and 28 a scanning circle whose center is determined by the intersection of the incident X-rays 22 with the galvanized annealed steel sheet 15 and whose radius is determined by the distance between the center and each detector 18, 19, 20 or 21. The detectors 18-21 (including the solar slits 27) are each provided with a drive motor 29. In addition, they are mounted on a common scanning plate 30, together with the associated motors 29, which is also provided with a drive motor 31. The detectors 18-21 (including the solar slits 27) can be driven independently or simultaneously on the scanning circuit 28 by the motors 29 for scanning purposes. More precisely, they can be driven simultaneously on the scanning circuit 28 by driving the scanning plate 30 with the motors 29. Reference numeral 32 represents a window formed of, for example, a polyimide film. In order to prevent corrosion of the X-ray tube 17, etc., a certain amount of air with little moisture or an inert gas such as nitrogen is introduced into and removed from the measuring unit 6 as an atmospheric gas. Also, the inside of the measuring unit 6 is set at a constant temperature by a temperature control unit 30. This is because a temperature change in the atmospheric gas brings about a density change in the atmospheric gas, which otherwise leads to a change in the absorption of the incident X-rays and the diffracted X-rays 23-26 by the atmospheric gas, that is, to a deterioration in the accuracy with which the X-ray diffraction intensities can be measured.

In the present system, the X-ray tube 17 and the detectors 18-21 (including the solar slits 27) are arranged in the same plane on the surface of the galvanized annealed steel sheet, which satisfies Bragg's formula represented by the following equation:

nλ = 2d sin Θ

wherein:

n is the reflection index,

λ is the wavelength of X-rays (10⊃min;¹⊃0;m) [(Å)]

d is the interplanar distance (10⊃min;¹⊃0;m) [(Å)] and

Θ is the Bragg angle (the diffraction angle of the X-rays) (degrees).

For scanning purposes, the detectors 18-21 (including the solar slits 27) are then driven independently or simultaneously within the ranges of the following interplanar distance d of the grating planes on the scanning circle 28, with the center being at the intersection of the incident X-rays 22 with the galvanized annealed steel sheet 15.

The scanning ranges of the detectors are shown as follows.

For the detector measuring the X-ray diffraction intensity of the -phase: the interplanar distance d of the lattice planes = 1.30x10⁻¹⁰ - 1.23x10⁻¹⁰m (1.30 Å-1.23 Å);

for the detector measuring the X-ray diffraction intensity of the δ1 phase: the interplanar distance d of the lattice planes = 1.32x10⁻¹⁰ - 1.25x10⁻¹⁰m (1.32 Å-1.25 Å);

for the detector for measuring the X-ray diffraction planes of the Γ phase: the interplanar distance d of the lattice planes = 1.25x10⁻¹⁰ - 1.20x10⁻¹⁰m (1.25 Å-1.20 Å); and

for the detector measuring the background intensity: the interplanar distance d of the lattice planes = 1.20x10⁻¹⁰ - 1.17x10⁻¹⁰m (1.20 Å-1.17 Å).

The interplanar distance d of the lattice planes of the -, δ₁- and Γ-phases to be measured with the present system is as follows:

d = about 1.26x10⁻¹⁰m (1.26 Å) for the phase;

d = about 1.28x10⁻¹⁰m (1.28 Å) for the δ₁ phase; and

d = about 1.22x10⁻¹⁰m (1.22 Å) for the Γ-phase.

While the X-ray diffraction peaks in case of interplanar spacing larger than the distances determined above, i.e. those of the -, δ₁- and Γ-phases, are slightly lower 2θ position have strong X-ray diffraction intensities, there is difficulty in separating the peaks from each other because they overlap each other. In order to measure an alloying degree with high accuracy, in other words, while limiting the influence on the accuracy of the X-ray diffraction intensities to be measured of a deviation of the galvanized annealed steel sheet 15 from the detectors 18-21 for the diffracted X-rays 23-26 due to its fluctuations, it is preferable to increase the angles 2θ of exit of the diffracted X-rays 23-26. This is because the interplanar distance d of the lattice planes of the -, δ₁ and Γ phases to be measured are fixed at the above-mentioned values at which the X-ray diffraction peaks do not overlap each other at all and the diffraction angles 2θ of X-rays are increased. The interplanar distance of lattice planes at the background measurement position is also fixed at the value defined above, at which no X-ray diffraction peak can be found.

Not all of the detectors 18-21 (including the solar slit 27) need to be driven on the same scanning circuit for scanning purposes. Where the scanning circuit 28 varies in radius from location to location, the distance between the galvanized annealed steel sheet 15 and the detectors 18-21 will vary, but they must be corrected for intensity for a difference in X-ray diffraction intensities. It is thus preferred that the detectors 18-21 be driven on the same scanning circuit for scanning purposes.

According to the system of the invention, it is to be noted that the detectors 18-21 can be fixed in place in the above-mentioned scanning area to measure the alloying degree.

The system for measuring X-ray diffraction intensities according to this invention operates in a stepwise scanning method, whereby the detectors 18-21 are driven stepwise every small angle within the above-mentioned range over a certain measuring period.

The measurement conditions are:

Step angle: 0.01º to 1.00º, and measurement time 1 to 100 seconds.

Although the type of the X-ray tube 17 used in the present system of the invention is not critical, the incident angle α and the diffraction angle 2θ thereof should preferably be sufficiently large to prevent deterioration in the accuracy of measurement by deviation of the diffracted X-rays 23-26 from the detectors 18-21 due to fluctuations of the galvanized annealed steel sheet 15. Thus, it is preferable to use as the X-ray tube 17 a Cr tube which produces a long wavelength and gives an increased angle 2θ. As the X-ray incident angle α increases, the distance of transmission of the diffracted X-rays 23-26 becomes long, the absorption of the diffracted X-rays 23-26 by the coating layer increases, resulting in a decrease in the X-ray diffraction intensities and hence in a deterioration in the accuracy of measurement. In contrast, as the X-ray incident angle α increases, the distance of transmission of the diffracted X-rays 23-26 becomes long, the absorption of the diffracted X-rays 23-26 by the coating layer increases, resulting in a decrease in the X-ray diffraction intensities and hence in a deterioration in the accuracy of measurement. In contrast, as the X-ray incident angle α increases, the diffraction angle 2θ increases. decreases, the distance of transmission of the diffracted X-rays 23-26 decreases, but the distance of transmission of the incident X-rays 22 increases, the absorption of the incident X-rays by the coating layer increases, which leads to a decrease in the X-ray diffraction intensities and thus to a deterioration in the accuracy of the measurement. As a result of investigations carried out by the present inventors on a Cr tube, it was found that the X-ray incidence angle α is preferably in the range of 60º to 75º, whereby the highest X-ray diffraction intensities are obtained.

While there is no particular limitation on the optical system used, it is to be noted that a parallel beam type optical system is preferred over a converging beam type optical system so as to limit the influence of deviation of the diffracted X-rays 23-26 from the detectors 18-21 due to fluctuations in the galvanized annealed steel sheet 15 on the accuracy of measurement of the X-ray diffraction intensities.

The alloying degree, i.e., the quality of the galvanized annealed steel sheets, can be determined by the X-ray diffraction intensities found from the measured X-ray diffraction profiles at the apex position of the X-ray diffraction peaks of the respective phase. In this case, however, the X-ray diffraction intensities must be corrected for the coating weight because they vary depending on the coating weight even if the alloying degree is at the same level. For this reason, another additional device for measuring the coating weight is required, which not only represents a considerable expense for maintaining the device, but also makes the calculation complicated.

Among the qualities of the galvanized annealed steel sheet 15, the amounts of the formed - and Γ phases are of interest and the following two values for the determination are preferably used, whereby the alloying degree can be accurately found from the X-ray diffraction and the background intensities of these two phases by a simple arithmetic operation, as stated in the specification of Japanese Patent Application No. 1-308917.

To determine the phase: I( ) - IB( )/I( )

To determine the Γ-phase: I(Γ - IB(Γ)/I(Γ)

where:

I ( ) is the total X-ray diffraction intensity of the - phase with an interplanar distance d = about 1.26x10⁻¹⁰m (1.26 Å) (c.p.s.),

I (Γ) is the total X-ray diffraction intensity of the - phase with an interplanar distance d = about 1.28x10⁻¹⁰m (1.28 Å) (c.p.s.), and

IB ( ) and IB (Γ) are the background intensities of the X-ray diffraction peaks of the - and Γ phases. However, in the present system, the X-ray diffraction intensity at an interplanar distance d = about 1.18x10-10m (1.18 Å), at which no X-ray diffraction peak is found, is conveniently used.

In addition, it is preferable that the accuracy of the measurement of the alloying degree can be further greatly improved by using, in combination with the above-mentioned values for determination, the following value for determination in which the X-ray diffraction and the background intensities of the - and δ1 phases are used as shown in Japanese Laid-Open Patent Publication No. 56-12314.

I( ) - IB( )/I(δ₁) - IB(δ)₁)

By using the present system using the former two values for determination, preferably together with the latter value for determination, it is possible to make an on-line determination of the quality of the galvanized annealed steel sheet 15, in particular the chalking property and the drawing property, which are now considered a serious problem, such as press formability.

According to the system of the present invention, wherein, as mentioned above, the detectors 18-21 are independently provided for detecting the X-ray diffraction and background intensities of the - and Γ phases, and more preferably for detecting the X-ray diffraction intensity of the δ1 phase, and are driven within the above-mentioned range for scanning purposes, a continuous on-line determination of the X-ray diffraction profiles within the range of interplanar spacing of about 1.30x10-10 to 1.17x10-10 m (1.30 Å to 1.17 Å) can be achieved within a short measurement time. In addition, it is possible to make an accurate determination of the X-ray diffraction intensities of the respective phases even if the vertex positions of the X-ray diffraction peaks vary with a change in the alloying degree. It is thus possible to make a continuous on-line determination of the quality of the galvanized annealed steel sheet 15 with improved accuracy. Note that in order to measure the determination values of the - and Γ phases, only the detectors for measuring the X-ray diffraction intensities of the - and Γ phases need be driven to effect scanning.

The system according to the present invention is applicable not only to galvanized annealed steel sheet 15 produced by the hot-dip method, but also to all kinds of galvanized annealed steel sheets obtained by electrogalvanizing and zinc vapor deposition, followed only by the heat treatments.

Examples

With the present system, as shown in Figures 4 and 5, the alloying degree of galvanized annealed steel sheet on a hot-dip galvanizing line of the non-oxidizing furnace type.

The conditions for measuring the alloying degree and producing galvanized annealed steel sheets are mentioned below.

The conditions for measuring the alloying degree:

X-ray tube: Cr target (with a wavelength Cr-kα₁ of approximately 2.29x10⊃min;¹⊃0;m (2.29 Å)

Optical system: parallel beam type

Tube voltage: 40 kV

Tube current: 70 mA

Filter: No filter for the X-ray tube side; V for the side of detection of the intensities of the -, δ₁- and Γ-phases, as well as for the background side

Solar slot: 0.6º

X-ray incidence angle: 60º

Areas exposed to X-rays: 650 mm² (width: 10 mm, length: 65 mm)

Detectors: Sealed type proportional counters Detector scanning ranges, where d is the interplanar distance:

Detector for measuring the X-ray diffraction intensity of the δ1 phase: d = 1.32x10⁻¹⁰-1.25x10⁻¹⁰m (1.32 Å-1.25 Å) (and 2θ = 120.5º - 132.5º)

Detector for measuring the X-ray diffraction intensity of the - phase: d = 1.30x10⁻¹⁰ - 1.23x10⁻¹⁰m (1.30 Å-1.23 Å) (and 2θ = 124.3º - 136.3º)

Detector for measuring the X-ray diffraction intensity of the Γ phase: d = 1.25x10⁻¹⁰ - 1.20x10⁻¹⁰m (1.25 Å-1.20 Å) (and 2θ = 133.0º - 145.0º)

Detector for measuring background intensity: d = 1.25x10⁻¹⁰ - 1.17x10⁻¹⁰m (1.25 Å-1.17 Å) (and 2Θ = 144.5º - 156.5º)

Distance between the X-ray tube and the galvanized annealed steel sheet: 300 mm

Distance between the galvanized annealed steel sheet and each detector: 500 mm

Predominant atmospheric gas in the measuring head: dry air (with a dew point of -10ºC and at a flow rate of 10 l/min.)

Internal temperature of the shielding chamber: 25ºC

Measuring method: step-by-step scanning

Step angle: 0.2º

Measuring time 1 second

To traverse the measuring head: the head is repeatedly moved back and forth in the width direction once per minute to measure three points on the galvanized annealed steel sheet, i.e. both sides of the width direction and the center each way.

Galvanizing and annealing conditions:

Type of base steel: Ti-added steel (C: 0.002%, Ti: 0.07%, Si: 0.15% and Mn: 0.13%)

Base steel size: Thickness: 0.7 mm and width: 1000 mm Zinc bath composition: 0.14 wt% Al/Zn

Zinc bath temperature: 480 ºC

Line speed: 50-150 m/min.

Base steel temperature in galvanizing and annealing furnace: 480- 600ºC

Coating weight: about 30-60 g/m² (by gas wiping method)

It should be noted that for the galvanizing and annealing, a galvanizing and annealing furnace was used, including a plurality of direct flame type burners at positions from the edge to the middle of each side of the steel sheet to be treated, while the amounts of gas, supplied to the burners were adjusted independently.

First, galvanized annealed steel sheets with coating weights of about 45 g/m2 processed in the galvanizing and annealing furnace at a constant line speed of 100 m/min but at different base steel temperatures of 480ºC, 520ºC and 600ºC were measured at their centers for X-ray diffraction profiles. In Fig. 6, the X-ray diffraction profiles of the galvanized annealed coating processed in the galvanizing and annealing furnace at the base steel temperatures of 480ºC, 520ºC and 600ºC are shown at 34, 35 and 36, respectively. It is found that variations in the sheet temperature in the galvanizing and annealing furnace give rise to changes in the X-ray diffraction intensities and the apex positions of the X-ray diffraction peaks of the -, δ₁ and Γ phases. The X-ray diffraction profiles shown in Fig. 6 were obtained at the step angle of 0.2° for the measurement time of 1 second, but the time required for the measurement was only 50 seconds. However, it takes 170 seconds to obtain the X-ray diffraction profiles within the same 2θ range as in Fig. 6 by driving a single detector.

With the system according to the present invention it is thus possible to obtain X-ray diffraction profiles within a time period that is about 1/3 shorter than that required when using a single detector; it is possible to obtain an on-line determination of X-ray diffraction profiles within a much shorter time period.

If the scanning ranges of the respective detectors are limited to the 2θ ranges of the X-ray diffraction peaks of the -, δ₁- and Γ- phases, the measurement time can be further reduced, e.g. a time span of 50 seconds was was required to obtain the X-ray diffraction profiles shown in Figure 6 because the entire range of 2θ = 121.5°-155.5° was covered. To measure only the X-ray diffraction intensities at the apex positions of the X-ray diffraction peaks of the -, δ₁ and Γ phases, the respective detectors can be driven within the following ranges.

For the detector measuring the X-ray diffraction intensity of the -phase: 2Θ = 129.0º-131.0º (and d = 1.27x10⁻¹⁰-1.26x10⁻¹⁰m) (1.27 Å-1.26 Å)

For the detector measuring the X-ray diffraction intensity of the Γ phase: 2θ = 138.0º-141.0º (and d = 1.23x10⁻¹⁰-1.21x10⁻¹⁰m) (1.23 Å-1.21 Å)

The time required to determine the X-ray diffraction intensities within the above-mentioned scanning areas is 20 seconds.

Then, using galvanized annealed steel sheets varying in alloying degree produced on a hot-dip galvanizing line at the base steel temperature in the galvanizing and annealing furnace and the coating weight, both of these factors being varied within the above-mentioned ranges, investigations were made into the relationship between the determination values of the - and Γ phases and the drawing property and the chalking property. The drawing property was determined in terms of the ratio of the outer diameter in such a drawing test method as schematically outlined in Figure 7, in which the same lubricating oil was used.

Conditions for the pull test:

Test piece

Diameter (D0) of the disc (galvanized annealed steel sheet) before drawing: 75 mm

Thickness of galvanized annealed steel sheet used for drawing: tmm

shape

Diameter (d) of the punch used for drawing: 40 mm Tip radius of the punch used for drawing: 5 mm Radius of the shoulder of the nozzle for drawing: 5 tmm Blank holder force applied during drawing: 1000 kg

Condition after the test

Drawing depth: 20 mm

Diameter of the flange after drawing: D�1 mm Ratio of the outer diameters: D�1/D�0

The chalking property was evaluated by such a chalking test method as outlined in Fig. 8. The conditions therefor were as follows: A test piece was subjected to a bend of 180º with the surface to be tested being inside, thereby forming on this surface a curvature with a diameter six times the thickness t of the test piece. After bending back, a cellophane tape was attached to the surface of the test piece and was peeled off therefrom for visual evaluation of the amount of powder of the coating metal deposited on this tape.

Rank 5: no powder was found

4: a small amount of powder was found

3: a noticeable amount of powder was found

2: a large amount of powder was found

1: a lot of powder was released even if no tape was used

Regarding the press formability of the test pieces, it was rated as good if it was up to a ratio of outer diameter of 0.745 or less in the pull test and rank 3 or more in the chalking test.

Table 1 shows results of the drawing test and the chalking test for the two coils of galvanized annealed steel sheets produced under the same galvanizing and annealing conditions. The determination values of the γ and Γ phases of one coil were found from the X-ray diffraction intensities thereof measured at the apices of the X-ray diffraction peaks by driving the detectors for scanning, while those of another coil were determined from the X-ray diffraction intensities thereof measured using the associated detectors mounted in the following positions.

For the detector measuring the X-ray diffraction intensity of the -phase: 2Θ = 130.3º (and d = 1.26x10⁻¹⁴m) (1.26 Å)) For the detector measuring the X-ray diffraction intensity of the Γ-phase: 2Θ = 139.0º (and d = 1.22x10⁻¹⁴m (1.22 Å)) Table 1

Remarks: The term "sampling" refers to the results obtained by driving the - and Γ phase detectors, and the term "fixed" refers to the results obtained by using the - and Γ phase detectors that were fixed in place.

From Table 1, it can be seen that in order to produce galvanized annealed steel sheets which satisfy both the drawing property and the chalking property, the method is carried out in such a way that the determination values of the - and Γ phases are about 0.35 or less and about 0.40 or less, respectively.

However, when the detectors for the - and Γ phases are fixed at the above-mentioned positions, it is impossible to obtain accurate measurements of the X-ray diffraction intensities of the - and Γ phases at the apex position of the X-ray diffraction peaks; the detection values of the - and Γ phases are lower than those obtained by driving the associated detectors. Particularly, in the case of Examples Nos. 1, 3 and 5 where the detectors are fixed in place, the detection values of - and Γ phases are within the above-mentioned appropriate range, e.g., 0.35 or less and 0.40 or less, respectively, without regard to deterioration of the press formability.

According to the system of this invention, wherein as shown in detail herein, detectors for detecting the X-ray diffraction intensities of the - and Γ phases and the background intensity are provided preferably together with a detector for detecting the X-ray diffraction intensity of the δ1 phase independently for scanning purposes, it is possible to determine the X-ray diffraction intensities of the - and Γ phases of the galvanized annealed steel sheet heat treated (for alloy formation) just after coating at the peak positions of the X-ray diffraction peaks and to measure the X-ray diffraction intensity of the δ1 phase at the peak position of the X-ray diffraction peak when the detector for detecting the X-ray diffraction intensity of the δ1 phase is additionally provided; it is possible to carry out a non-destructive and continuous on-line determination of the press formability of the galvanized annealed steel sheets with improved accuracy. Thus, the alloying degree measured by this system can be directly fed back to the heat treatment (alloying) conditions to put them under appropriate control, whereby improved galvanized annealed steel sheets can be stably produced in quality without causing quality change in the line direction of the galvanized annealed steel sheets. This entails that any outgoing (outside the production line) inspection can be dispensed with, contributing to energy saving, reducing production costs, and the like. It is also possible to obtain a rapid and easy determination of the alloying degree of existing galvanized annealed steel sheets. The present invention is therefore of great industrial value.

Claims (3)

1. System for on-line determination of the alloying degree in galvanized annealed steel sheets (15), which comprises an X-ray tube (17) for generating X-rays, a detector (19, 20) for measuring the X-ray diffraction intensity of the -phase and Γ-phase in intermetallic Fe-Zn compounds of this galvanized and annealed coating, and a detector (18) for measuring the background intensity, all of these detectors (18, 19, 20) and the X-ray tube (17) being arranged in the same plane on the surface of galvanized annealed steel sheets (15), and which further comprises an arithmetic unit (10) for receiving the calculation results of the X-ray diffraction intensities, wherein the detector (20) for measuring the X-ray diffraction intensity of the -phase, the detector (19) for measuring the X-ray diffraction intensity of the Γ-phase and the detector (18) for measuring the background intensity are independently or simultaneously within the respective interplanar distance ranges of d = 1.30x10⁻¹⁰m - 1.23x10⁻¹⁰m (1.3 Å-1.23 Å), d = 1.25x10⁻¹⁰m - 1.20x10⁻¹⁰m (1.25 Å-1.20 Å) and 1.20x10⁻¹⁰ - 1.17x10⊃min;¹⊃0;m (1.20 Å-1.17 Å) and on a circumference whose center is defined by the intersection point of the incident X-rays with the galvanized annealed steel sheets (15). 2. System according to claim 1, characterized in that it further comprises a detector (21) located in the same plane on the surface of said annealed galvanized steel sheet (15) for measuring the X-ray diffraction intensity of the δ1 phase, the X-ray diffraction intensity being input into said arithmetic unit (10). 3. System according to claim 2, characterized in that the detector (21) for detecting the X-ray diffraction intensity of the δ1 phase is driven independently or simultaneously with the first-mentioned three detectors (18, 19, 20) within the interplanar distance range of d = 1.32x10⁻¹⁰m - 1.25x10⁻¹⁰m (1.32 Å - 1.25 Å) and on a circumference whose center is defined by the intersection point of the incident X-rays with the galvanized annealed steel sheet (15).